Tandem Genetic Duplications. The mechanism by which tandem genetic duplications arise in bacteriophage lambda is being studied. Two independent methods of detecting and assaying duplication mutants have been developed. The effects of mutations inactivating the phage-specific and also the host- specific recombination systems have been studied. The tandem duplications are mapped by electron microscopy of DNA heteroduplexes. Present results indicate that tandem duplications in lambda do not arise by errors in ordinary recombination and that there are no special endpoints for the duplications. Mechanism of Protein Folding. The reversible unfolding reaction of ribonuclease A (with disulfide bonds intact) is being studied by fast-reaction techniques and by NMR spectroscopy. The kinetics of thermal unfolding and refolding are biphasic, indicating that at least three species are involved. Both the fast and slow refolding reactions yield native enzyme as product. The unfolding enzyme contains different, fast-refolding and slow-refolding, species. These species are interconvertible in a pH-dependent equilibrium. Both species are present at temperatures above the transition zone for thermal unfolding. Many of the kinetic results can be explained by a simple, 3-species kinetic mechanism. Work is in progress to find out if this mechanism is applicable to the reversible unfolding reactions of other small proteins.